The solar interior cannot be observed directly. Our knowledge is
based on mathematical models. The solar interior can be separated into three
large zones. Energy is generated in the core (1) by nuclear reactions. The conversion of hydrogen
to helium is the dominant nuclear reaction (97% of radiated solar energy).
About half billion tuns of hydrogen is changed to helium every
second. Nuclear reactions are highly sensitive to temperature and density.
Two individual hydrogen nuclei must collide with enough energy
to overcome the repulsive electrical force between positively charged nuclei
and to form helium nucleus. The temperature at the very center of the Sun
is about 15,600,000 K and the density is approximately 150 g/cm3.
The temperature drops to half its central value at distance 175,000 km from
the center (about 25% of the distance from center to the photosphere) and the
density drops to about 1/8 of its central value. Nuclear reactions practically
shut off and therefore the edge of the solar core is defined at this distance.
The energy generated in the solar core is at first transported by means of
radiation. The so called radiative zone (2) extends up to 70% of the distance
from the solar center to the photosphere. The energy is
carried by photons (mostly gamma-rays and x-rays) which bounce from particle
to particle through the radiative zone. Although the photons travel at the
speed of light they bounce so many times that an individual photon takes
about a million years to come through the radiative zone. The temperature
of radiative zone bottom is about 7,000,000 K, at the top "only" 2,000,000 K.
The density drops down from 20 g/cm3 to only 0.2 g/cm3.
The opacity of the solar material increases with distance from the solar center
and at the top of radiative zone the opacity is so hight that the transport of
energy by radiation is ineffective. The high opacity is a consequent of relatively
"low temperature". Heavier atoms are not totally ionized and they are able to hold some of
their electrons which makes the material more opaque. Convection is the method of energy transport
in this opaque material (ascending and descending flows - "boiling motion").
The convection zone (3) is the outer-most layer of solar interior. It extends from a depth
of about 200,000 km up to the visible surface. The temperature drops in this
layer from 2,000,000 K down to 5,700 K and the density from 0.2 g/cm3
drops down to 0.0000002 g/cm3. The radiative and convective zone are separated
by so-called interface layer. It is now believed that the solar magnetic field is
generated in this layer.
